1. Field of the Invention
Surface impregnated catalyst, process for the production thereof, and use thereof for the preparation of vinyl acetate.
2. Description of the Prior Art
It is known that vinyl acetate can be prepared in the gas phase from ethylene, acetic acid and oxygen; the supported catalysts used for this synthesis comprise palladium and an alkali metal element, preferably potassium. Further additives used are cadmium, gold or barium.
In the Pd/K/Au catalysts both noble metals are generally applied in the form of an impregnated layer on the support; they are produced by impregnation and subsequent precipitation of the metal salts by means of alkaline compounds (U.S. Pat. No. 4,048,096, U.S. Pat. No. 3,775,342).
In Pd/K/Ba catalysts the metal salts are applied by impregnation, spraying on, vapor deposition, dipping or precipitation (EP-A-0 519 436). The same methods are known for Pd/K/Cd catalysts (U.S. Pat. No. 4,902,823; U.S. Pat. No. 3,393,199, U.S. Pat. No. 4,668,819). Furthermore, the production of a Pd/K/Au or Pd/K/Cd surface impregnated catalyst is known, with a specific support material being washed with an acid prior to impregnation and being treated with a base after impregnation (EP-A-0 519 435).
The German Patent Application P 42 11 780.1 describes obtaining Pd/K/Au, Pd/K/Ba or Pd/K/Cd catalysts built up in the form of an impregnated layer by atomizing a solution of corresponding metal salts by means of ultrasound and then, in a limited amount and over a limited time applying this to the support particles and commencing the drying thereof in such a way that the catalytically active metal salts cannot penetrate into the core of the support particles, but only into an outer part having a greater or lesser thickness, the impregnated layer.